linux-kernel - Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

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Date: Wed, 31 Jan 2024 11:43:07 +0100
From: David Hildenbrand <david@...hat.com>
To: Yin Fengwei <fengwei.yin@...el.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
 Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
 Michael Ellerman <mpe@...erman.id.au>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

On 31.01.24 03:20, Yin Fengwei wrote:
> On 1/29/24 22:32, David Hildenbrand wrote:
>> This series is based on [1] and must be applied on top of it.
>> Similar to what we did with fork(), let's implement PTE batching
>> during unmap/zap when processing PTE-mapped THPs.
>>
>> We collect consecutive PTEs that map consecutive pages of the same large
>> folio, making sure that the other PTE bits are compatible, and (a) adjust
>> the refcount only once per batch, (b) call rmap handling functions only
>> once per batch, (c) perform batch PTE setting/updates and (d) perform TLB
>> entry removal once per batch.
>>
>> Ryan was previously working on this in the context of cont-pte for
>> arm64, int latest iteration [2] with a focus on arm6 with cont-pte only.
>> This series implements the optimization for all architectures, independent
>> of such PTE bits, teaches MMU gather/TLB code to be fully aware of such
>> large-folio-pages batches as well, and amkes use of our new rmap batching
>> function when removing the rmap.
>>
>> To achieve that, we have to enlighten MMU gather / page freeing code
>> (i.e., everything that consumes encoded_page) to process unmapping
>> of consecutive pages that all belong to the same large folio. I'm being
>> very careful to not degrade order-0 performance, and it looks like I
>> managed to achieve that.
> 
> One possible scenario:
> If all the folio is 2M size folio, then one full batch could hold 510M memory.
> Is it too much regarding one full batch before just can hold (2M - 4096 * 2)
> memory?

Good point, we do have CONFIG_INIT_ON_FREE_DEFAULT_ON. I don't remember 
if init_on_free or init_on_alloc was used in production systems. In 
tlb_batch_pages_flush(), there is a cond_resched() to limit the number 
of entries we process.

So if that is actually problematic, we'd run into a soft-lockup and need 
another cond_resched() [I have some faint recollection that people are 
working on removing cond_resched() completely].

One could do some counting in free_pages_and_swap_cache() (where we 
iterate all entries already) and insert cond_resched+release_pages() for 
every (e.g., 512) pages.

-- 
Cheers,

David / dhildenb